I am still struggling with this, however, I am making progress.  I have tracked this issue down to my Moog Slim Phatty, and it does not seem to be USB ground loop related.
 
Here are the symptoms - on the Focusrite interface, I turn up the input on the Moog.  I get a wicked buzz from the monitors.  This happens whether or not the Moog's power is actually plugged in.  
 
As soon as I unplug the TRS cable from the Moog, the noise simply disappears.  I have tried different cables, but it has no effect.   Would balanced vs. unbalanced cables have an effect?  The output of the Moog is unbalanced, but the cable is a balanced cable.  (I don't have an unbalanced cable to test).
 
I do not have the same issues with any of my other synths, and I have changed inputs on the Focusrite as well, and the problem follows the Moog.
 
I guess I don't know where to go from here. 

husker
As soon as I unplug the TRS cable from the Moog, the noise simply disappears.  I have tried different cables, but it has no effect.   Would balanced vs. unbalanced cables have an effect?  The output of the Moog is unbalanced, but the cable is a balanced cable.  (I don't have an unbalanced cable to test).
 

if that's the case, a simple DI box plugged between the Moog and the interface should fix it

Also maybe have an electrician check your AC power outlets to make sure the grounding in your music room is proper.  From the pics I see that the Moog has a direct AC main in, not a wall wart DC transformer.

Abacab, I would tend to agree, but the noise happens whether or not the Moog is actually plugged into the wall or not.  I can have the power totally unplugged, and the noise still occurs.
 
This is driving me insane, so I spent a few hours in the middle of the night on this.  I also seem to have the problem on the "left" output of my Nord.   The "right" output does not experience the issue.  Both outs of the FA are OK as well.
 
Basically, if I turn the the input up on the Focusrite on either of those two "inputs" I get that wicked buzz - it matters not if the power is actually plugged in or not.  As soon as I unplug the cable, the noise disappears.

And this thread...
 
Little Phatty very strange problem (ground loop??)
https://www.gearslutz.com...oblem-ground-loop.html

Back when I mainly used external synth modules, I ran them all through a small Mackie audio mixer first, then into my audio interface.  That seemed to prevent he sort of issues you are describing, except for a weird humming we used to get from my buddies Telecaster.  Not sure if we ever totally solved that one, but Rob's suggestion of a DI box might be the answer.

Thank you.  Would it make sense that only one of the Nord "outs" experiences the issue?  Would I need an "active" or "passive" DI box?

I've never had to invest in a DI, but my understanding is that you would want "active" for anything that needed a pre-amp.
 
Since your Moog should be outputting at instrument level, passive would most likely work.

Thanks Abacab.  I think I am going to throw in the towel on this.  The cheapest solution I guess is simply just to turn down those inputs on the Focusrite when I don't need them.  I'm just one of these guys who wants to understand everything, and the whole thing has just really bothered me.
 
This issue has really gotten me sidetracked on what I want to do - learn about Sonar and make some music.

Since you do have several external pieces of gear, you might consider investing in a good used hardware mixer to connect all hardware modules outboard of the audio interface. 
 
This compact mixer has very flexible routing options.  I have an ancient version of this that served me well, back in the day.  It has a second stereo bus with the Mute/Alt 3-4 buttons in the channel strip.  This is routed to a separate pair of outputs, so you really can have four outs at a time (Main 1-2 & Alt 3-4).  Plus with control room outs, you can run directly to a set of monitors or an amp without needing a computer to play live.
 
For example this Mackie 1202 new is only $270. 
https://www.amazon.com/Mackie-1202VLZ4-12-Channel-Compact-Mixer/dp/B00EDHVQN6/ref=dp_ob_title_ce
 
On eBay, pre-owned seems to run about $100-150.
https://www.ebay.com/p/Mackie-1202-VLZ-Pro-Mixer-1202vlz-1202VLZPRO-12-U076692/1519581409